The present invention relates to the protection of stator cores from overflux damage and, more particularly, to the prevention of harmful interlaminar currents within the laminations of stator cores of dynamoelectric machines.
Typically, the stator cores of electrical generators are constructed with a multiplicity of punched laminations stacked to form a cylindrical core with a central bore therethrough. The inside diameter of the cylindrical core has a plurality of axially extending slots which are formed by stacking the laminations with their prepunched teeth aligned in a predetermined manner. The prepunched laminations are made from thinly rolled stock and are coated with a generally nonconductive inorganic surface. The two planar surfaces of each punching are arranged to be perpendicular to the central axis of the cylindrical stator core.
During operation, lines of magnetic flux run generally from one pole of the generator rotor radially outward into the stator core, circumferentially through a portion of the stator core and then radially inward to another pole of the rotor. As the rotor revolves, the above-described flux path rotates and induces an alternating electrical current in the stator coils which are disposed within the axially extending stator core slots described above.
However, near the two axial ends of the stator core, the lines of flux do not behave exactly as described above but, instead, tend to fringe axially outward from the two ends of the rotor field winding and reenter the stator core at right angles to the planar surfaces of the laminations at each axial end of the stator core. To protect the laminated punchings from damage that may be caused by this axially impinging flux, generator stators are generally protected by a stator end shield which consists of stepped laminations into which the fringing flux lines can be contained and provided with a circumferential path around the stator. Under normal conditions, the end-shield prevents these lines of flux from entering the stator core.
This end shield does not, however, provide complete protection during overvoltage or overflux conditions. Here, the laminated punchings located at the end of the main body of the stator core are subjected to lines of flux which pass through them in a direction which is perpendicular to their major planes. This flux induces intralaminar current paths which are most severe in the tooth region of the laminations which is located at the inside diameter of the stator core, closest to the rotor field winding. During this type of overflux operation, the intralaminar currents can cause overheating of the laminations' teeth or result in interlaminar faults. These faults can aggravate the above-described conditions and result in the tooth region of several axially adjacent laminations being electrically shorted together.
When this shorting occurs, a potential electrical path is created along a stator slot in the axial direction. Of course, the laminations are also electrically conductive within themselves. Since stator cores are typically built with conductive building bolts that run axially along the core's outside surface, a complete electrical current is created when the inside edges of two or more laminations are shorted. Following the creation or this type of short, the normal flux lines which run circumferentially around the core induce currents which can then travel axially along the short, radially outward through a lamination, axially along the building bolt and radially inward through another lamination, returning to the shorted region.
It should be apparent to one skilled in the art that, once these shorted circuits are created, normal operating flux which runs circumferentially around the core will aggravate the shorted condition and progressively produce severe damage to the stator core and ultimately require an expensive shutdown and repair of the electrical generator.
The present invention prevents this type of severe overflux damage by eliminating one leg of the above-described harmful current path. Specifically, an insulative cylinder is disposed about the building bolt in the region where these currents are most likely to arise. This prevents electrical communication between the building bolt and the laminations and thus, even in the event of shorted laminations in the tooth region of the laminations, no return path through the building bolt is available in which the damaging current can flow. In order to maintain the axial position of this insulative cylinder, two additional layers of non-magnetic plates are disposed within the laminated core, one on each axial end of this region of the core most likely to experience the axially impinging lines of flux. These plates are thicker than the laminations and are shaped to receive the insulative cylinder. The plates and the punchings therebetween are cooperatively associated to permit the stator core to be axially compressed without deforming the insulative cylinder.
It should be apparent that the present invention provides a means to minimize core damage caused by overflux conditions, to which electrical generators are occasionally subjected, by preventing the flow of harmful currents through the stator core.